Nutrient enrichment alters seasonal β-diversity in global grasslands

Intra-annual (i.e. seasonal) temporal niche partitioning is essential to the maintenance of biodiversity in many plant communities. However, understanding of how climate and global change drivers such as eutrophication influence seasonal niche partitioning in plant assemblages remains limited. We used early-season and late-season compositional data collected from 10 grassland sites around the world to explore relationships between climate variability and intra-annual species segregation (i.e. seasonal β-diversity) and to assess how nutrient enrichment alters seasonal β-diversity in plant communities. We then assessed whether changes in seasonal β-diversity in response to nutrient enrichment are underpinned by species turnover or nestedness and determined how specific functional groups (i.e. annual forbs, perennial forbs, C3 and C4 graminoids and legumes) respond to eutrophication within and across early and late sampling dates. We found a positive relationship between intra-annual temperature variability and seasonal β-diversity but observed no relationship between intra-annual precipitation variability and seasonal β-diversity. Nutrient enrichment increased seasonal β-diversity and increased turnover of species between early- and late-season communities. Nutrient enrichment reduced the abundance of C4 graminoids and legumes within and across sampling timepoints and eliminated intra-annual differences in these groups. In contrast, nutrient enrichment resulted in seasonal differences in C3 graminoids, which were not observed in control conditions and increased abundance of C3 graminoids and annual forbs within and across early and late sampling dates. Synthesis: Our understanding of how grasslands respond to various components of global change is primarily based on studies that document community changes at inter-annual scales. Using early-season and late-season compositional data from 10 grassland sites around the world, we show that nutrient enrichment increases seasonal β-diversity and alters intra-annual dynamics of specific functional groups in unique ways

Catching Element Formation In The Act

Gamma-ray astronomy explores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays and relativistic-particle acceleration, and the evolution of galaxies. MeV gamma-rays provide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nuclear de-excitation, and positron annihilation. The substantial information carried by gamma-ray photons allows us to see deeper into these objects, the bulk of the power is often emitted at gamma-ray energies, and radioactivity provides a natural physical clock that adds unique information. New science will be driven by time-domain population studies at gamma-ray energies. This science is enabled by next-generation gamma-ray instruments with one to two orders of magnitude better sensitivity, larger sky coverage, and faster cadence than all previous gamma-ray instruments. This transformative capability permits: (a) the accurate identification of the gamma-ray emitting objects and correlations with observations taken at other wavelengths and with other messengers; (b) construction of new gamma-ray maps of the Milky Way and other nearby galaxies where extended regions are distinguished from point sources; and (c) considerable serendipitous science of scarce events -- nearby neutron star mergers, for example. Advances in technology push the performance of new gamma-ray instruments to address a wide set of astrophysical questions.Comment: 14 pages including 3 figure

Regional Contingencies in the Relationship between Aboveground Biomass and Litter in the World’s Grasslands

Based on regional-scale studies, aboveground production and litter decomposition are thought to positively covary, because they are driven by shared biotic and climatic factors. Until now we have been unable to test whether production and decomposition are generally coupled across climatically dissimilar regions, because we lacked replicated data collected within a single vegetation type across multiple regions, obfuscating the drivers and generality of the association between production and decomposition. Furthermore, our understanding of the relationships between production and decomposition rests heavily on separate meta-analyses of each response, because no studies have simultaneously measured production and the accumulation or decomposition of litter using consistent methods at globally relevant scales. Here, we use a multi-country grassland dataset collected using a standardized protocol to show that live plant biomass (an estimate of aboveground net primary production) and litter disappearance (represented by mass loss of aboveground litter) do not strongly covary. Live biomass and litter disappearance varied at different spatial scales. There was substantial variation in live biomass among continents, sites and plots whereas among continent differences accounted for most of the variation in litter disappearance rates. Although there were strong associations among aboveground biomass, litter disappearance and climatic factors in some regions (e. g. U. S. Great Plains), these relationships were inconsistent within and among the regions represented by this study. These results highlight the importance of replication among regions and continents when characterizing the correlations between ecosystem processes and interpreting their global-scale implications for carbon flux. We must exercise caution in parameterizing litter decomposition and aboveground production in future regional and global carbon models as their relationship is complex

Resource-enhancing global changes drive a whole-ecosystem shift to faster cycling but decrease diversity

Many global changes take the form of resource enhancements that have potential to transform multiple aspects of ecosystems from slower to faster cycling, including a suite of both above- and belowground variables. We developed a novel analytic approach to measure integrated ecosystem responses to resource-enhancing global changes, and how such whole ecosystem slow-to-fast transitions are linked to diversity and exotic invasions in real-world ecosystems. We asked how 5-year experimental rainfall and nutrient enhancements in a natural grassland system affected 16 ecosystem functions, pools and stoichiometry variables considered to indicate slow versus fast cycling. We combined these metrics into a novel index we termed "slow-fast multifunctionality" and assessed its relationship to plant community diversity and exotic plant dominance. Nutrient and rainfall addition interacted to affect average slow-fast multifunctionality. Nutrient addition alone pushed the system towards faster cycling, but this effect weakened with the joint addition of rainfall and nutrients. Variables associated with soil nutrient pools and cycling most strongly contributed to this antagonistic interaction. Nutrient and water addition together, respectively, had additive or synergistic effects on plant trait composition and productivity, demonstrating divergence of above- and belowground ecosystem responses. Our novel metric of faster cycling was strongly associated with decreased plant species richness and increased exotic species dominance. These results demonstrate the breadth of interacting community and ecosystem changes that ensue when resource limitation is relaxed

Grazing and light modify Silene latifolia responses to nutrients and future climate

Altered climate, nutrient enrichment and changes in grazing patterns are important environmental and biotic changes in temperate grassland systems. Singly and in concert these factors can influence plant performance and traits, with consequences for species competitive ability, and thus for species coexistence, community composition and diversity. However, we lack experimental tests of the mechanisms, such as competition for light, driving plant performance and traits under nutrient enrichment, grazer exclusion and future climate. We used transplants of Silene latifolia, a widespread grassland forb in Europe, to study plant responses to interactions among climate, nutrients, grazing and light. We recorded transplant biomass, height, specific leaf area (SLA) and foliar carbon to nitrogen ratio (C:N) in full-factorial combinations of future climate treatment, fertilization, grazer exclusion and light addition via LED-lamps. Future climate and fertilization together increased transplant height but only in unlighted plots. Light addition increased SLA in ambient climate, and decreased C:N in unfertilized plots. Further, transplants had higher biomass in future climatic conditions when protected from grazers. In general, grazing had a strong negative effect on all measured variables regardless of added nutrients and light. Our results show that competition for light may lead to taller individuals and interacts with climate and nutrients to affect traits related to resource-use. Furthermore, our study suggests grazing may counteract the benefits of future climate on the biomass of species such as Silene latifolia. Consequently, grazers and light may be important modulators of individual plant performance and traits under nutrient enrichment and future climatic conditions

Regression of <i>Silene</i> height and vegetation and litter cover.

Regression of the visually estimated (a) total vegetation cover and Silene height and (b) litter cover and Silene height. The line represents a regression line with a 95% CI. The regressions are significant (a) F1,74 = 7.58, P = 0.007 (b) F1,56 = 21.74, P (TIF)</p

Treatment effects on <i>Silene</i> performance.

Height (a, n = 80) and biomass (b, n = 78) responses of Silene latifolia to combinations of grazing, fertilization, ambient and future climate and light addition. The data are means ± SE. Note, that two transplants were not harvested for biomass determination because they were grazed to a few millimeters from the ground.</p

Main and interactive treatment effects on <i>Silene</i> performance.

Main and interactive treatment effects on Silene performance.</p

Main and interactive treatment effects on <i>Silene</i> foliar traits.

Main and interactive treatment effects on Silene foliar traits.</p

Treatment effects on <i>Silene</i> foliar traits.

Foliar C:N (a, n = 36) and SLA (b, n = 32) responses of Silene latifolia to combinations of fertilization, ambient and future climate and light addition. The data are from inside the fences and represent means ± SE. Note, that there are less than 40 samples per trait, because some transplants did not have healthy and undamaged leaves.</p